Repetitive and/or lengthy exposure to static magnetic fields and static magnetic field gradients occurs occupationally in a number of professions, including hospital technicians working with magnetic research imaging (MRI), researchers in experimental high energy physics, and workers in aluminium smelters, for example. While static magnetic fields are recognised as being far less harmful than ionising radiation, repeated exposure to strong static fields is generally treated with advised caution from regulatory bodies and recommendations exist to limit occupational exposure. For example, The National Radiation Protection Board (NRPB) of the United Kingdom guidelines suggests limiting the time-weighted exposure to 0.2 T per 8-hour working day. The guidelines permit higher exposures of 2 T per working day if only extremities, but not the head or trunk, are exposed to the fields.
Furthermore, when a human body moves through static magnetic fields, particularly those that vary in space (as most do), currents are induced in the body. The human body, in electrical terms is effectively a heterogeneous array of conductive dielectric tissues. These currents can disturb biological function (see, for example, John F. Schenck, “Safety of Strong, Static Magnetic Fields”, J. Magn. Reson. Reson. Imag. 12: 2-19, 2000).
Dosimetry for ionizing radiation (such as X-rays) is already known and developed. A number of electromagnetic dosimeters have been described in prior art. For example, U.S. Pat. Nos. 4,672,309 and 6,486,664 describe devices to monitor radiofrequency electromagnetic exposure, that is, high frequency exposure.
Exposure to static magnetic field gradients can occur either when a person moves through a static magnetic field that is non-uniform in space or when the static magnetic field is switched from one value to another. Combinations of both these effects are also possible. The fields in and around an MRI magnet provide one example of spatial variation of magnetic fields. An MRI magnet has a volume, usually in the centre of the structure, that generates strong and very uniform static magnetic fields, but strong, long term gradients in the magnetic field exist outside this central region. Some patients experience uncomfortable sensations when moved into an MRI scanner or when moving their head during entry to the scanner, or once in the MRI scanner. Reported sensations include a feeling of falling, magnetophosphenes, a loss of proprioreception, a metallic taste in the mouth or muscle twitching (peripheral nerve stimulation).
Similar effects have been described when a field gradient is switched. Field gradients are devices used in MRI machines that generate linear field gradients in the z-component of the static magnetic field. These gradients are used to spatially encode the NMR signal and produce images. They are regularly switched during the creation of an image.
Movement through spatially varying magnetic fields can induce significant eddy currents in the body (L. Feng, H. Zhao and S. Crozier, “Induced fields by body movement and head-shake in High-Field MRI”, J. Magn. Reson. 161/1 pp 99-107, 2003). It is useful to examine the nature of such field inductions in the body, and a calculation of induced fields in the body and example simulations of motion around a 4 Tesla MRI magnet system can be found in S. Crozier and L. Feng, “Numerical evaluation of the fields induced by body motion in or near high-field MRI scanners”, Progress in Biophysics and Molecular Biology, Volume 87, Issues 2-3, February-April 2005, Pages 267-278.
Generally, this simulation shows that field magnitudes induced by typical body movements are less than, but of a similar magnitude to, those induced by switched gradient coils in the MRI scenario and have potential for adverse health effects.
There are many types of the known devices for measuring magnetic field strength, variously known as magnetometers, teslameters, gaussmeters and fluxmeters. Some of these are not portable or otherwise not adapted for ambulatory applications. Hence, they are unsuitable for measuring personal exposure to magnetic fields by persons who may be constantly moving around magnets in the course of their work.
Those magnetometers which are portable commonly use Hall effect sensors or “Hall probes” to sense magnetic fields, as such probes can be manufactured in small sizes. However, a disadvantage of many Hall probes is that they have limited dynamic range due to saturation. Consequently, they are unable to measure or record high field strengths accurately. Although there are some Hall probes with wide operating ranges, they tend to be bulky, expensive and lack resolution at low field strengths.
Most magnetometers are designed to measure instantaneous field strength, and do not record cumulative exposure to magnetic fields.
Further, as outlined above, changes in field strength, such as caused by switched or time-varying magnetic fields or movement through static field gradients, may also have detrimental effects on the human body. Known magnetometers are generally unable to provide an adequate or satisfactory measurement and/or record of changes in magnetic field experienced by a person.
It is an object of this invention to provide an improved method and apparatus for measuring and recording instantaneous and/or cumulative personal exposure to magnetic fields.